The glycation of haemoglobin and serum proteins is increased in patients with diabetes mellitus. The increase is dependent on the glucose concentration and the incubation period of the protein with glucose. In these cases the serum proteins, including haemoglobin, are not glycated enzymatically but rather by means of an uncatalysed chemical reaction of glucose with amino groups of proteins. Experts assume that the concentration of a particular protein-glucose adduct reflects the glucose concentration over a particular period as well as the turn-over rate of the protein. Glycated haemoglobin is regarded as an indicator of the average blood glucose concentration during the last two to three months before the blood collection and examination. Glycated serum protein shows the blood glucose concentration during a shorter period of time. The determination of glycated protein such as glycated haemoglobin (HbA.sub.1c) or glycated serum protein is therefore considerably important for the long-term glycemic control of diabetes patients.
In order to examine blood for the content of glycated protein the sample must often be transported to a far distant laboratory. The content of glycated protein in the sample should not change during this transport period and during a possible subsequent waiting period. The examination of blood samples which had been stored for a long period for glycated haemoglobin is reported in Clinical Chemistry 29, 1080-1082 (1983). This shows that whole blood can be stored up to 21 days at room temperature with essentially no change in the HbA.sub.1c content.
However, the transport of liquid blood samples is complicated and involves risks such as breakage of the transport vessel. In addition the puncture of a vein is necessary to collect whole blood although the small amounts obtained by withdrawing capillary blood from the finger pad would be sufficient for the analysis. Thus methods have been developed for the transport and analysis of smaller amounts of sample in which capillary blood is applied to filter paper and allowed to dry there. The filter paper is subsequently transported to the site of the examination. Here a disk containing the sample is cut out from the filter paper, eluted and the eluate is examined. The report in Clinical Chemistry 28, 386-387 (1982) refers to such a method. In this report it is stated that the content of glycated protein changes considerably compared to the original sample during blood sample storage on filter paper. After storage of whole blood on filter paper considerably increased measured values for glycated protein are found.
The impregnation of filter paper with glucose oxidase to prevent the increase in the content of glycated haemoglobin caused by storage of blood on filter paper is described in Clinical Chemistry 32, 869-871 (1986). However, impregnation with glucose oxidase was not able to completely prevent the increase of glycated protein. The false increase in the values can only be reduced by this measure. A further disadvantage of impregnating with glucose oxidase is its own instability during storage under the usual storage conditions.
Similar conclusions are reached by an article in Diabetes Care 10, 352-355 (1987). Here it is reported that the treatment of filter paper with glucose oxidase or with ethanol cannot satisfactorily prevent a false increase in the values for glycated haemoglobin when blood is stored on filter paper.
Apart from the poor stability of the sample, a further disadvantage of the methods described in the state of the art for the transport and storage of sample materials is that the liquid sample has to be completely dried before the final packaging. For this the filter paper has to be typically dried for 10 to 60 minutes in the air. Incompletely dried samples can lead to non-reproducible test results or for example contaminate the shipping packaging.